39、单例模式(下)
1、单例的唯一性
单例类在老进程中存在且只能存在一个对象,在新进程中也会存在且只能存在一个对象,而且这两个对象并不是同一个对象
这也就说:单例类中对象的唯一性的作用范围是进程内的,在进程间是不唯一的
2、线程唯一单例
public class IdGeneratorThread {
private IdGeneratorThread() {
}
private static final ConcurrentHashMap<Long, IdGeneratorThread> INSTANCES = new ConcurrentHashMap<>();
public static IdGeneratorThread getInstance() {
long currentThreadId = Thread.currentThread().getId();
INSTANCES.putIfAbsent(currentThreadId, new IdGeneratorThread());
return INSTANCES.get(currentThreadId);
}
private final AtomicLong id = new AtomicLong(0);
public long getId() {
return id.incrementAndGet();
}
}
3、集群环境下的单例
我们需要把这个单例对象序列化并存储到外部共享存储区(比如文件)
进程在使用这个单例对象的时候,需要先从外部共享存储区中将它读取到内存,并反序列化成对象,然后再使用,使用完成之后还需要再存储回外部共享存储区
为了保证任何时刻在进程间都只有一份对象存在,一个进程在获取到对象之后需要对对象加锁,避免其他进程再将其获取
在进程使用完这个对象之后,还需要显式地将对象从内存中删除,并且释放对对象的加锁
public class IdGenerator {
private AtomicLong id = new AtomicLong(0);
private static IdGenerator instance;
private static SharedObjectStorage storage = FileSharedObjectStorage(/*入参省略, 比如文件地址*/);
private static DistributedLock lock = new DistributedLock(); // 分布式锁
private IdGenerator() {
}
public synchronized static IdGenerator getInstance() {
if (instance == null) {
lock.lock();
instance = storage.load(IdGenerator.class);
}
return instance;
}
public synchronized void freeInstance() {
storage.save(this, IdGeneator.class);
instance = null; // 释放对象
lock.unlock();
}
public long getId() {
return id.incrementAndGet();
}
}
4、多例
// 后端服务器
public class BackendServer {
private long serverNo;
private String serverAddr;
private static final int SERVER_COUNT = 3;
private static final Map<Long, BackendServer> serverInstances = new HashMap<>();
static {
serverInstances.put(1L, new BackendServer(1L, "192.168.135:8080"));
serverInstances.put(2L, new BackendServer(2L, "192.168.136:8081"));
serverInstances.put(3L, new BackendServer(3L, "192.168.137:8082"));
}
private BackendServer(long serverNo, String serverAddr) {
this.serverNo = serverNo;
this.serverAddr = serverAddr;
}
public BackendServer getInstance(long serverNo) {
return serverInstances.get(serverNo);
}
private static final Random RANDOM = new Random();
public BackendServer getRandomInstance() {
return serverInstances.get((long) RANDOM.nextInt(SERVER_COUNT) + 1);
}
}
本文来自博客园,作者:lidongdongdong~,转载请注明原文链接:https://www.cnblogs.com/lidong422339/p/17505734.html